Cette étude porte sur une technologie de densification des boues appliquée à un procédé à boues activées continue. Par la combinaison d'une sélection microbienne (zone anaérobie) et physique (extraction sélective par hydrocyclones), cette technologie permet de générer une proportion significative de biomasse sous forme granulaire, améliorant ainsi la vitesse de sédimentation de la suspension. L'objectif est ici d'évaluer numériquement le potentiel d'un procédé utilisant cette technologie pour répondre à des scénarios de conception et d'exploitation plus contraintes : augmentation de la concentration en boues dans le bassin d'aération (panne d'extraction, problème sur l'évacuation des boues), réduction du taux de recirculation, hypothèses plus agressives quant aux vitesses ascensionnelles praticables. Pour ce faire, le clarificateur objet de l'étude a été modélisé en mécanique des fluides numériques, en intégrant les paramètres de sédimentation des boues densifiées. Ces simulations permettent d'évaluer la réponse de l'ouvrage en matière d'évolution du voile de boues et d'épaississement de la suspension. Les résultats obtenus démontrent le maintien des performances de clarification à forte concentration de travail (6 gMES/L), ou en appliquant un taux de recirculation très faible (15 %), avec une boue densifiée pouvant s'épaissir jusqu'à environ 30 gMES. /L. Également, la boue densifiée normalement concentrée permet d'admettre des surcharges hydrauliques jusqu'à 1,0 m/h de manière continue, ou de 1,5 à 2,0 m/h si le taux de recirculation est fixé à 30 et 32 % respectivement. Les résultats obtenus lorsque la surcharge hydraulique avoisine les 2 m/h sont à prendre avec précaution, car la concentration en matières en suspension (MES) de sortie prédite est proche de la limite réglementaire (24 mgMES/L). Globalement, les résultats de cette étude indiquent la possibilité de reconsidérer de manière plus optimale le dimensionnement des clarificateurs secondaires et du procédé à boues activés lors de la mise en œuvre de la densification.

There have been significant advances in the use of biological and physical selectors for the intensification of continuously flowing biological wastewater treatment (WWT) processes. Biological selection allows for the development of large biological aggregates (e.g., mobile biofilm, aerobic granules, and densified biological flocs). Physical selection controls the solids residence times of large biological aggregates and ordinary biological flocs, and is usually accomplished using screens or hydrocyclones. Large biological aggregates can facilitate different biological transformations in a single reactor and enhance liquid and solids separation. Continuous-flow WWT processes incorporating biological and physical selectors offer benefits that can include reduced footprint, lower costs, and improved WWT process performance. Thus, it is expected that both interest in and application of these processes will increase significantly in the future. This review provides a comprehensive summary of biological and physical selectors and their design and operation.

Conventional activated sludge (CAS) and densified sludge obtained using hydro-cyclone selective wasting were compared at a full-scale water resources recovery facility. The densified tested sludge, containing around 30-50% of aerobic granules, showed enhanced settleability with low and stable sludge volume index (SVI) compared to CAS, which suffered recurrent filamentous bulking. Further in-depth batch settling tests were carried out using a 40 cm diameter column fitted with ultrasonic transducers to monitor both sludge blanket height and vertical velocity profiles. Hindered settling and compression parameters were calibrated from the experiment for latter modelling use. Test sludge displayed more than doubled settling velocities compared to CAS, with hindered settling velocities remaining >3 m·h-1 even at high solids concentrations of 6.85 g·L-1. The compression regime was attained at much higher critical concentration for the test sludge. It also displayed enhanced thickening properties, with concentrations obtained after 30 min of settling being 20.9 and 8.5 g·L-1 respectively for test and control sludge. This allows for a substantial reduction of recirculation rates in practice. These results open perspectives in optimizing existing plant operation as well as clarifier design and modelling using densified sludge.

Full-scale demonstration of activated sludge conversion into a granule-floc hybrid process was implemented in Dijon (France) water resource recovery facility (WRRF). Biomass densification was achieved based on external gravimetric selection using hydrocyclones within continuous-flow anaerobic-anoxic-oxic (A(2)O) biological nutrient removal (BNR) bioreactor. The goal was to optimize settleability of biological sludge by lowering and stabilizing sludge volume index (SVI) to improve process robustness and resiliency. Process proved to stabilize operation and to uncouple the total solids residence time (SRT) between floc and granule morphologies. The densified biomass initially produced stable SVI < 100 ml/g for a period of 4 months and thereafter a steady state year-round SVI below 50 ml/g, including the winter period during which the control train SVI expansion >200 ml/g. The densified biomass successfully broke the vicious cycle of interannual bulking. Form and function interrelationship is proposed for the densified biomass (hybrid floc-granule). The concept of biological architecture is proposed as the purposeful control of granule and floc proportions, with a proposed "form factor" ratio of 1:2 granule to floc, that produce a "SRT uncoupling function factor" ratio of 4:1 granule to floc, further resulting in very stable settling and effluent functionalities. Practitioner Points Controlling granule-floc proportions allows for sludge volume index (SVI) operational adjustment, which further allows for increased clarified design accuracy. One-third aggregates dramatically improved settling characteristics: 20% and 35% of AGS ensures SVIs below 100 and 50 ml/g, respectively. Densified biomass enables new SRT and clarifier flux rates approaches for engineering and operation practices: Doubling typical surface loading rates from 6.0-8.5 to 15-20 kg m(-2) h(-1) and surface overflow rates from 0.6-0.8 to 1.5-2.4 m/h SRT uncoupling of 1:4 is achieved between floc and granule, enabling specific niche environment for fast and slow growing organisms.

Industrial full-scale application of high-rate dissolved air floatation (DAF) in the municipal wastewater treatment plant (WWTP) of Grenoble (France) has highlighted outstanding performance results leading to new design-to-cost perspectives. The integration of DAF technology to treat the returns from the backwash waters of submerged biological aerated filters (BAF) (nitrification stage) has demonstrated removal efficiencies that allow further room for global process optimization. The results obtained on nitrifying BAF backwash water showed a DAF outlet water concentration of less than 25 mg.L−1 of total suspended solids at 25 m.h−1, with only polymer conditioning. Such high clarification performance allows leveraging of valuable cost optimization of global process design integration. Direct discharge from DAF's outlet into the receiving body can be implemented. Hydraulic and solid return loads can therefore be significantly reduced at the inlet of the WWTP. Moreover, floated sludge extracted from the DAF units achieved 4.4% dryness on average. The high thickening operational performance of this DAF technology is able to produce sludge directly compatible with anaerobic digestion. These full-scale results demonstrate that Suez's GreenDAF™-BWW technology in such application can leverage new rooms for design improvement for BAF treatment and total cost optimization of both the mainstream water treatment line and sludge line.